The present invention relates generally to methods for making absorbent sheet and more particularly to a wet-crepe process utilizing a narrow crepe shelf configuration.
Wet crepe processes for making absorbent sheet are known in the art, for example, there is disclosed in U.S. Pat. No. 3,432,936 to Cole et al. a wet-crepe, throughair dry process wherein a web is creped off a Yankee cylinder and subsequently throughdried on an impression fabric. While various conditions may be employed, creping is carried out at 40 percent consistency (Example. 1) and drying rates approach 28 pounds of water removed/hour-ft2 (Example 2).
U.S. Pat. No. 4,356,059 to Hostetler is directed to a system for producing absorbent sheet wherein the web is creped from a first creping surface, passes through a nip formed between a dewatering felt and a printing fabric and is applied to and creped from a second creping surface. According to the ""059 patent, the web has a consistency of from about 40 to 50 percent after creping from the first rotating heated cylinder. See Column 3, lines 5-15.
U.S. Pat. No. 5,851,353 to Fiscus et al. discloses a method for can drying wet webs for tissue products to preserve wet bulk. In one embodiment, the patent describes restraining a wet-creped web between a pair of sheet molding fabrics. The restrained wet web is processed over a plurality of can dryers to dry the wet web, for example, from a consistency of from about 40 percent to a consistency of at least about 70 percent. The sheet molding in the fabrics protect the web from direct contact with the can dryers and impart an impression in the web. A can drying assembly and tissue machine for accomplishing the method are also disclosed. Prior to being restrained between the pair of molding fabrics, the wet web most typically has a consistency of from about 50 to about 60 percent. See Column 6, lines 1-5.
There is provided in accordance with the present invention a method of making absorbent sheet including the steps of: (a) depositing an aqueous cellulosic furnish on a foraminous support; (b) at least partially dewatering the furnish to form a nascent web; (c) applying the nascent web to a rotating cylinder and drying the web to a consistency of from about 30 to about 90 percent solids; (d) creping the web at the consistency of from about 30 to about 90 percent while maintaining a narrow creping shelf about a creping surface of a creping blade; and (e) drying the creped web to form the absorbent sheet, wherein the absorbent sheet exhibits a Void Volume of at least about 3.5 gms/gm. The process is advantageously applied to absorbent sheet having a thickness of from about 0.003 inches to about 0.010 inches. A preferred blade has creping ledge widths of from about 0.005 to about 0.025 inches.
In one embodiment, water may be pressed out of the sheet in a conventional cold press, either a roll or shoe press and the sheet is creped off one of the press rolls in accordance with the present invention. The sheet may then be molded into a fabric and dried in a single tier can section arrangement to produce a high porosity sheet.
As used herein the term percent or % refers to weight percent and the term consistency refers to weight percent of fiber unless the context indicates otherwise.
As used herein, xe2x80x9cVoid Volumexe2x80x9d is determined by saturating a sheet with a nonpolar liquid and measuring the volume of liquid absorbed. The volume of liquid absorbed is equivalent to the Void Volume within the sheet structure. The Void Volume is expressed as grams of liquid absorbed per gram of fiber in the sheet structure. More specifically, for each single-ply sheet sample to be tested, 8 sheets are selected and cut out a 1 inch by 1 inch square (1 inch in the machine direction and 1 inch in the cross-machine direction). For multi-ply product samples, each ply is measured as a separate entity. Multiply samples should be separated into individual single plies and 8 sheets from each ply position used for testing. Weigh and record the dry weight of each test specimen to the nearest 0.0001 gram. Place the specimen in a dish containing POROFIL(trademark) liquid, having a specific gravity of 1.875 grams per cubic centimeter, available from Coulter Electronics Ltd., Northwell Drive, Luton, Beds, England; Part No. 9902458.) After 10 seconds, grasp the specimen at the very edge (1-2 millimeters in) of one corner with tweezers and remove from the liquid. Hold the specimen with that comer uppermost and allow excess liquid to drip for 30 seconds. Lightly dab (less than xc2xd second contact) the lower comer of the specimen on #4 filter paper (Whatman Ltd., Maidstone, England) in order to remove any excess of the last partial drop. Immediately weight the specimen, within 10 seconds, recording the weight to the nearest 0.0001 gram. The Void Volume for each specimen, expressed as grams of POROFIL per gram of fiber, is calculated as follows:
Void Volume=[W2xe2x88x92W1)/W1],
wherein
xe2x80x9cW1xe2x80x9d is the dry weight of the specimen, in grams; and
xe2x80x9cW2xe2x80x9d is the wet weight of the specimen, in grams.
The Void Volume for all eight individual specimens is determined as described above and the average of the eight specimens is the Void Volume for the sample.